Use a drysuit. These insulate, are far warmer than a wetsuit and are an additional source of buoyancy.
Two varieties, neoprene & 'membrane'. Typically neoprene is cheaper so your first drysuit is neoprene then you move on to membrane.
To cut to the chase: membrane means there's little insulation, but there's hardly any evaporation so it's much warmer on the surface than a neoprene drysuit -- such as on a RIB motoring at speed back from a dive. All insulation is gained by your multiple layers of underclothes. This is great as you can change your undergarments to suit the conditions -- don't forget to adjust your weighting as you need more weight to counteract the additional buoyancy of the insulation. Membrane also doesn't 'squeeze' so doesn't change it's buoyancy characteristics during a dive as does a wetsuit or neoprene drysuit.
I "cheat" in the winter and use a (Santi) heated vest fed from a dedicated large 18AH battery through an E/O connector into the vest. You won't be warm, but you're certainly not shivering, especially important if you're decompressing. Also use drygloves with decent inner gloves, two (or three) layers of socks and a thick hood. Did a 100 minute dive in a 2 degree lake this winter and only boredom got me out of the water -- I had 5 layers on top: a thermal vest; thermal T-shirt; the heated vest; a long-sleeved rash-vest; a thick winter insulated long-sleeved top (Fourth Element Arctic Expedition).
I also use the same membrane drysuit in warmer condtions; it's great at 24 degrees with a thin layer of insulation. In fact I regularly practice with my drysuit in a 32degree heated pool.
If buying a drysuit, make sure it's fitted with large pockets on the sides. Also make sure the suit exhaust dump is on the side of the arm, not the front (which some manufacturers seem to think is easier to get to, not realising that you don't touch the valve during a dive - once you set it to wide open).
Then there's the solution to excess hydration: a "P" valve -- the second most valuable invention in diving after the demand valve! There's nothing worse than having to cut short a dive to keep the drysuit dry inside.
Some things to consider about drysuits:
- There's drysuit training courses. There's not much to learn but you must practice with them as there's issues with buoyancy. The difficult bit is the top 10 metres. It's soon mastered (tip, leave the dump open!)
- Most shops will throw in the drysuit course for free if you buy the suit
- You don't need a 'ticket' to buy a drysuit so don't pay another dollar in
- Fit is everything. Well, after being dry! You must be able to stretch upwards, crouch into a ball, stand up and reach your spine with your hand from over your shoulders (for valve shutdowns), lay down and bend your knees at 90 degrees (the standard flat diving position); make sure there's enough space for your thick winter undersuit.
- If you're not a standard size consider made-to-measure but make sure that they measure you so if it doesn't fit, it's their responsibility
- Expensive drysuits aren't necessarily better than cheaper ones: some brands are very overpriced
- Zips: back or front. Front's more expensive but way more convenient.
- Zips: plastic zips break and leak more easily than metal zips
- Boots or socks: I much prefer socks with separate rock boots as I can keep the muck away from my suit after walking across sand or carpark. Socks means the suit can be turned inside out for speedy drying.
- Socks+rockboots stop gas migration to your feet, so you don't get that floaty-feet feeling
- Buy a decent bag and ensure you fold the suit up properly every time
- Dry the suit thoroughly after every dive or it will stink (I hang mine up in the garage)
- Use a "HangAir" which is a big hanger with a fan to blow air into the suit for drying
- Cuffs and neck seals: neoprene is warmer and more comfortable, but tears easily and is hard to replace. Latex or silicone seals stretch more and and easier to don and doff
- Premium suits have user-replacable neck and cuff seals (Scitec Quick Neck) (Kubi glove rings)
- Membrane dries much quicker than neoprene
- Inflation nipple: use a standard BCD type so you can use the same hoses (there's some other types - CENJ - which aren't compatible)
- P-valves can be easily fitted after you've bought the suit. Once fitted they can't be easily removed (glue)
- Buying one second hand can be good value but be very careful about fit and leaking. You need a guarantee from the person selling it that it won't leak and it will fit. Be warned, you could be throwing away a huge amount of money on a suit that leaks and doesn't fit. If I were buying one, I would only do this on a sale-or-return basis -- I've seen far too many friends buying teabags. The only real way of finding out if it leaks is to do a long dive with dry underclothes.
- Second-hand suits are not worth much unless they're in very good condition with a guarantee/warranty.